Why is Eugene FM signal have different Horizontal and Vertical Polarization. For example, KMGE have 49kw V and 21 KW H. Why is that?

Author: Kennewickman
Monday, April 27, 2009 - 9:19 am

Some engineer decided that there were more FM antennas out there that are vertically polarized. It is called circular polarization in total, so that the Effective Radiated Power ( ERP ) is some value depending upon the license they have for an ERP value .

So this value can be expressed like they gave it above in a horizontal and vertical component. It is not a simple addition of the two values it is an algebraic sum and different antenna designs for that sum can be diffeerent values in the vertical and horizontal plane.

Author: Radionut
Monday, April 27, 2009 - 10:39 am

What he said.

Author: Kent_randles
Monday, April 27, 2009 - 12:29 pm

It's actually elliptical polarization.

Author: Broadway
Monday, April 27, 2009 - 12:44 pm

show off...

Author: Jr_tech
Monday, April 27, 2009 - 12:47 pm

Right hand, Left hand, or not rotating elliptical?

Author: Kent_randles
Monday, April 27, 2009 - 5:52 pm

Chris "Ichabod" Murray, engineer for several Eugene stations, is responsible for it. I have no idea which way it rotates, but it does rotate.

Pete Onnigan, one of the founders of Jampro, used to explain a circularly polarized signal as being like a 2-bladed propellor, spinning through space.

"In 25 words or less, how is eliptical different from circular polarization?"

Circularly polarized signals are a special case of elliptical polarization because the E vector not only rotates, but also the magnitude remains unchanged.

23 words.

The different ERP at H and V is the giveaway.

Author: Semoochie
Tuesday, April 28, 2009 - 12:41 am

I'm sorry, I misread Kennewickman's point. I thought Kent was saying that the same horizontal and vertical power could ALSO be and in this case, WAS eliptical. I withdraw the question and thank you for the answer.

Author: Tomparker
Tuesday, April 28, 2009 - 10:16 am

I prefer the flux capacitor.

Author: Jr_tech
Tuesday, April 28, 2009 - 10:47 am

What caught my eye in the Wikipedia reference above was this statement:

"FM radio

The term "circular polarization" is often used erroneously to describe mixed polarity signals used mostly in FM radio (87.5 to 108.0 MHz), where a vertical and a horizontal component are propagated simultaneously by a single or a combined array. This has the effect of producing greater penetration into buildings and difficult reception areas than a signal with just one plane of polarization. This would be an instance where the polarization would more appropriately be called random polarization (or simply unpolarized). See Stokes parameters."

I thought that modern FM transmitting antennas mostly produce circular polarization... is this not the case?

Author: Andy_brown
Tuesday, April 28, 2009 - 12:42 pm

Mostly.

Previously, 1/4 wave spaced separate horizontal and vertical elements were used to achieve circular. In the NCE band, TV Ch 6 protection has up until this point forced many licensees to lower horizontal power or abandon horizontal output altogether, so you still see separate elements in their antennae. The wikipedia entry could be better written. Most C.P. antennae can be recognized because the horseshoe shaped elements are tilted, not perfectly horizontal.

> In 25 words or less, how is eliptical different from circular polarization?

In circular polarization, the horizontal and vertical components of the signal must be of equal strengths. (16 words)

Early FM stations used horizontal polarization, just like TV stations. They broadcast from cloverleaf or turnstile antennas. This worked great with rooftop antennas (which have the elements arranged horizontally) or indoor wire antennas that could be laid out or tacked up on a wall.

When FM moved into the automobile, there was a problem: at the time, cars had vertical whip antennas. In theory, a vertically oriented whip like that is not going to pick up a horizontally polarized signal. In practice, these antennas picked up weak signals with lots of multipath (when radio signals bounce off objects, the polarization of the reflected signal can come off a little cock-eyed, depending on the shape and orientation of the object reflecting the signal).

Engineers solved this problem with new antenna designs that simultaneously transmitted horizontally and vertically polarized signals. Instead of producing fields that look like sinewaves, these antennas radiate fields that look like corkscrews. Even an antenna with a random orientation, like the headphone cord in Walkman radio, can work satisfactorily with this type of a signal.

I believe (correct me if I'm wrong), that the FCC rules do not permit full-powered FM stations to broadcast only a vertically polarized signal. LPFMs and translators are allowed to do this, in some cases. As an alternative, some broadcasters go with radiating most of the power in the vertical plane, as KMGE is doing.

Author: Jr_tech
Tuesday, April 28, 2009 - 1:21 pm

A few years ago I played with a CP antenna for FM Dxing to achieve more gain, and possibly provide better rejection of signals from the back, which should have the opposite sense of rotation. Results were very mixed... I concluded:

1. There is perhaps NO standard rotation sense for FM broadcasting.

2. After a few bounces, a DX FM station (100 + miles) ends up being mostly Horizontal, at least in this area. (might be different in New York City)

Author: Alfredo_t
Tuesday, April 28, 2009 - 2:25 pm

> A few years ago I played with a CP antenna for FM Dxing

COOL! Did you combine the outputs of two antennas to get a circularly polarized response? Or, did you build the antenna from scratch?

Without getting too far off topic, I have toyed a little bit with the idea of building circularly polarized 2.4 GHz array and using that to build a 386server.info public Internet access point. What has kept me from undertaking this is the potential liabilities of sharing my Internet connection with the public.

After the poor/inconsistent results from that experiment, I tried just one in vertical polarization, which lead me to conclusion #2.

Author: Andy_brown
Tuesday, April 28, 2009 - 4:43 pm

"(correct me if I'm wrong), that the FCC rules do not permit full-powered FM stations to broadcast only a vertically polarized signal. "

Wrong. Lots of full power NCE's operate just that way.

Author: Jr_tech
Wednesday, April 29, 2009 - 12:01 pm

Can the Horizontal component from a vertical antenna REALLY be zero ?

Author: Andy_brown
Wednesday, April 29, 2009 - 12:43 pm

Now you're splitting hairs. Alfredo is referring to being licensed at zero horizontal ERP, which is allowed and prevalent in the NCE band because of Ch. 6 interference. In fact, it's the only way a lot of NCE's get on the air when the Ch 6 in question won't provide a waiver. Of course, it's all going to pretty much moot here shortly, but the rules are still on the books. In '07 I did lots of NCE apps for their FCC window to apply that had less H than V and some that were no H at all.

Sure, there is going to be some errant V that could be measured in the H plane, but really now ...

I thought I read somewhere that sat radio is using different H and V ERP, but haven't looked into that. Maybe Kent knows, or maybe someone else can google around with it. I thought I read that V from sat has lower multipath then H, but it might be some other reason.

Moving antennae (the geostationery satellite is sort of fixed, but vehicles are moving) are quite complicated. The EM math is totally over the top.

Author: Semoochie
Friday, May 15, 2009 - 1:34 am

For the third time, 1320AM has applied to triplex with 840 and 1450, while reducing power to 600 watts. This goes back to 2002, when they first applied. In each case, the construction permit has run out before the station was built. The previous time, they re-applied the following day after the CP expired.